What’s Vulnerable › Vulnerabilities

CVE-2026-27448: pyOpenSSL allows TLS connection bypass via unhandled callback exception in set_tlsext_servername_callback

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-18 · updated 2026-06-17
• Weakness: CWE-636

Description

pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 0.14.0 and prior to version 26.0.0, if a user provided callback to `set_tlsext_servername_callback` raised an unhandled exception, this would result in a connection being accepted. If a user was relying on this callback for any security-sensitive behavior, this could allow bypassing it. Starting in version 26.0.0, unhandled exceptions now result in rejecting the connection.

Affected products

• pyca pyopenssl: from 0.14 before 26.0.0 — fixed in 26.0.0

How to fix

Upgrade to a fixed version: pyca pyopenssl 26.0.0. Check the vendor advisory for mitigations.

References

• https://github.com/pyca/pyopenssl/blob/358cbf29c4e364c59930e53a270116249581eaa3/CHANGELOG.rst#L27
• https://github.com/pyca/pyopenssl/commit/d41a814759a9fb49584ca8ab3f7295de49a85aa0
• https://github.com/pyca/pyopenssl/security/advisories/GHSA-vp96-hxj8-p424

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